Editing Skittle Chromatography (section)

== Why It Is ==

TLC is a simple, quick, and inexpensive procedure that gives the chemist a quick answer as to how many components are in a mixture. A TLC plate is a sheet of glass, metal, or plastic which is coated with a thin layer of a solid adsorbent (usually silica or alumina). A small amount of the mixture to be analyzed is spotted near the bottom of this plate.

The TLC plate is then placed in a shallow pool of a solvent in a developing chamber so that only the very bottom of the plate is in the liquid. This liquid, or the eluent, is the mobile phase, and it slowly rises up the TLC plate by capillary action.

As the solvent moves past the spot that was applied, an equilibrium is established for each component of the mixture between the molecules of that component which are adsorbed on the solid and the molecules which are in solution.

In principle, the components will differ in solubility and in the strength of their adsorption to the adsorbent and some components will be carried farther up the plate than others. When the solvent has reached the top of the plate, the plate is removed from the developing chamber, dried, and the separated components of the mixture are visualized. If the compounds are colored, visualization is straightforward.

The strength with which an organic compound binds to an adsorbent depends on the strength of the following types of interactions: ion-dipole, dipole-dipole, hydrogen bonding, dipole induced dipole, and van der Waals forces. With silica gel, the dominant interactive forces between the adsorbent and the materials to be separated are of the dipole-dipole type.

Highly polar molecules interact fairly strongly with the polar Si—O bonds of these adsorbents and will tend to stick or adsorb onto the fine particles of the adsorbent while weakly polar molecules are held less tightly.

Weakly polar molecules thus generally tend to move through the adsorbent more rapidly than the polar species. The stationary phase is a powdered adsorbent which is fixed to a aluminum, glass, or plastic plate. The mixture to be analyzed is loaded near the bottom of the plate. The plate is placed in a reservoir of solvent so that only the bottom of the plate is submerged.

This solvent is the mobile phase; it moves up the plate causing the components of the mixture to distribute between the adsorbent on the plate and the moving solvent, thus separating the components of the mixture so that the components are separated into separate "spots" appearing from the bottom to the top of the plate.

In Thin Layer chromatography, the stationary phase (the adsorbent: silica gel or alumina) is polar, and the polarities of both the component of the mixture and the solvent used as the mobile phase are the determining factors in how fast the compound travels. 

TLC is usually used only to analyze mixtures: to determine the number of components and to see if a desired component is present.